What are the management and treatment approaches for patients with Right Bundle Branch Block (RBBB) versus Left Bundle Branch Block (LBBB) on an electrocardiogram (ECG)?

ECG Findings in RBBB vs LBBB

Electrocardiographic Characteristics

Right Bundle Branch Block (RBBB)

• QRS duration ≥120 ms with terminal R wave in lead V1 (classic RSR' or "rabbit ears" pattern) 1
• Wide, slurred S waves in lateral leads (I, aVL, V5-V6) 1
• Normal or rightward QRS axis in most cases 2
• ST-segment and T-wave changes are typically discordant (opposite direction to terminal QRS deflection) 3

Left Bundle Branch Block (LBBB)

• QRS duration ≥120 ms with broad, notched R waves in lateral leads (I, aVL, V5-V6) 2
• Absence of Q waves in lateral leads (I, aVL, V5-V6) 1
• Absence of S waves in leads I and aVL is characteristic 1
• Precordial S/T wave ratio <1.8 is consistent with "new LBBB" pattern 2
• Inferior QRS axis is common, particularly in acute or symptomatic LBBB 2
• Discordant ST-segment and T-wave changes (opposite to QRS direction) 4

Clinical Significance and Management Implications

LBBB: Higher Risk Profile

LBBB carries significantly greater clinical implications than RBBB and requires more aggressive evaluation. 5

• Transthoracic echocardiography is mandatory (Class I recommendation) in all newly detected LBBB to exclude structural heart disease, as LBBB markedly increases the likelihood of left ventricular systolic dysfunction 5, 6
• LBBB is strongly associated with coronary artery disease and heart failure development, unlike RBBB 5
• Advanced imaging (cardiac MRI, CT, or nuclear studies) is reasonable if echocardiogram is unrevealing but structural disease is still suspected 5, 6
• Stress testing with imaging may be considered in asymptomatic patients when ischemic heart disease is suspected 5, 6

RBBB: Lower Risk Profile

• Echocardiography is reasonable (Class IIa) if structural heart disease is suspected, but the threshold for imaging is lower than with LBBB 5
• RBBB has increased risk of left ventricular dysfunction compared to normal ECGs, but significantly less than LBBB 5
• RBBB patients had 64% increased odds of in-hospital death in acute MI settings, compared to 34% for LBBB, suggesting RBBB may indicate more extensive disease when MI occurs 5

Critical Diagnostic Pitfalls

Acute Myocardial Infarction Detection

Both LBBB and RBBB obscure ST-segment analysis, making acute MI diagnosis extremely challenging. 5, 4

• Traditional ST-elevation criteria are unreliable in the presence of bundle branch blocks 4
• LBBB particularly masks ischemic changes because ST-segment elevation is common in LBBB without acute ischemia 4
• Sgarbosa criteria are highly specific (can rule in MI) but too insensitive to rule out MI in LBBB patients 4
• Approximately 10% of LBBB patients with acute MI present without typical symptoms 6
• Both RBBB and RBBB patients are significantly undertreated for acute MI, with only 32% of RBBB and 16.7% of LBBB patients receiving fibrinolytic therapy compared to 65.5% without bundle branch block 5

Alternating Bundle Branch Block

Alternating bundle branch block (QRS complexes alternating between LBBB and RBBB morphologies) requires immediate permanent pacing (Class I recommendation) due to high risk of sudden complete heart block 5

Pacing Indications

When Permanent Pacing IS Indicated

• Syncope with bundle branch block AND HV interval ≥70 ms or infranodal block on electrophysiology study (Class I) 5
• Alternating bundle branch block (Class I) 5
• Symptoms suggesting intermittent AV block warrant diagnostic evaluation and possible pacing 5

When Permanent Pacing IS NOT Indicated

Asymptomatic patients with isolated LBBB or RBBB and 1:1 atrioventricular conduction should NOT receive permanent pacing (Class III: Harm recommendation) 5

Cardiac Resynchronization Therapy

• CRT may be considered (Class IIb) in heart failure patients with LVEF 36-50%, LBBB with QRS ≥150 ms, and Class II or greater symptoms 5, 6

Monitoring Strategy

Symptomatic Patients

• Ambulatory ECG monitoring is useful (Class I) in symptomatic patients with conduction system disease when AV block is suspected 5
• Electrophysiology study is reasonable (Class IIa) in patients with symptoms suggestive of intermittent bradycardia (lightheadedness, syncope) when ambulatory monitoring is unrevealing 5

Asymptomatic Patients

• Ambulatory monitoring may be considered (Class IIb) in asymptomatic patients with extensive conduction disease to document suspected higher-degree AV block 5, 6
• Serial ECGs should monitor for progression of conduction disease 6

Key Clinical Pearls

• LBBB with precordial S/T ratio <1.8 and inferior axis suggests acute or "new" LBBB pattern, which may indicate acute pathology 2
• Painful LBBB syndrome is a rare entity causing chest pain from intermittent LBBB without ischemia, characterized by very low S/T ratio and inferior axis 2
• LBBB can coexist with coronary disease, complicating chest pain assessment 2
• False-positive admission rates increase by 50% in patients with ECG-LVH or bundle branch blocks presenting with ACS symptoms 7
• 30-day mortality is 3.5 times higher in patients with ECG-LVH compared to those without these abnormalities 7